Air separation with work expansion to high and low pressure rectification stages

ABSTRACT

A process for cryogenic separation wherein precleaned compressed air is divided into a first minor portion which is cooled and work expanded to relatively low pressure, a second minor portion liquefied and distilled, and a major portion work expanded to relatively high pressure.

United States Patent Coveney et al.

[ Sept. 16, 1975 AIR SEPARATION WITH WORK EXPANSION TO HIGH AND LOW PRESSURE RECTIFICATION STAGES Inventors: Joseph F. Coveney, North Tonawanda; Louis M. La Clair, Grand Island, both of N.Y.

Assignee: Union Carbide Corporation, New

York, N.Y.

Filed: July 21, 1971 App]. No.: 164,831

Related US. Application Data Continuation-impart of Ser. No. 849,376, Aug. 12,

1969, abandoned, and a continuation-in-part of Ser. No. 126,642, March 22, 1971, abandoned.

US. Cl. 62/13; 62/18; 62/38 Int. Cl. F25J 5/20 Field of Search 62/13, 14, 15, 23, 24,

References Cited UNITED STATES PATENTS Rouge 62/ 2 9 2,413,752 1/1947 Dennis 62/38 2,655,796 10/1953 Rice 62/38 2,822,675 2/1958 Grenier... 62/38 2,964,914 12/1960 Schuftan 62/38 3,100,696 8/1963 Becker 62/39 3,210,948 10/1965 Schilling. 62/38 3,214,925 11/1965 Becker 62/38 3,312,073 4/1967 Jackson 62/38 3,375,674 4/1 968 Becker 62/38 3,718,005 2/1973 McDermott 62/38 Primary Examiner-Norman Yudkoff Assistant ExaminerFrank Sever Attorney, Agent, or FirmJ. C. LeFever [57] ABSTRACT A process for cryogenic separation wherein precleaned compressed air is divided into a first minor portion which is cooled and work expanded to relatively low pressure, a second minor portion liquefied and distilled, and-a major portion work expanded to relatively high pressure.

10 Claims, 3 Drawing Figures SHEET 1 OF 2 L'IQUID NITROGEN PRODUCT LIQUID OXYGEN PRODUCT PRECLEANED AIR FEED INVENTORS JOSEPH F. COVENEY LOUIS M. LA CLAIR ATTORNEY PAIEIIIII'J I 5 III-75 PRODUCT 28 HEATER FOR REMOVAL 0F ACETYLENE AND CARBON DIOXIDE INVENTORS JOSEPH F. COVENEY OUIS M. LA CLAIR ATTORNEY 1 3 AIR SEPARATION WITH WORK EXPANSION TO HIGH AND LOW PRESSURE RECTIFICATION STAGES i CROSS REFERENCE TO RELATED APPLICATIONS This application is a continuation-in-part of both applications Ser. No. 849,376 filed Aug.. 12, 1969 and Ser. No. 126,642 filed Mar. 22, 1971 each filed in the names of Joseph F. Conveney and Louis M. LaClair, both now abandoned.

BACKGROUND OF THE INVENTION This invention relates to the cryogenic separation of air, and more particularly concerns a process for the low temperature separation of air into one or more liquid products and one or more gaseous products.

For the production of liquid products, both low pressure plants and their associated independent liquefiers, and high pressure plants, have well recognized limitations. Low pressure plants with liquefiers have poor thermodynamic efficiency, and require complex and expensive equipment. High pressure plants, which necessarily require reciprocating compressors and expanders rather than turbines, are restricted by high cost, and generally poor mechanical reliability. Also, reciprocating compressors and expanders are usually not oil free, and thus result in expensive and troublesome cry ogenic processing. More importantly perhaps, the high 2000-2500 psig. pressures at the feed heat exchanger of a high pressure separation plant require heavy-duty shell-and-tube type heat exchangers, rather than the smaller less expensive plate-and-fin exchangers permissible in low pressure plants.

The possibility of a mid-pressure compromise between a low and a high pressure plant is a deceptive answer to the problem of producing a liquid oxygen or nitrogen product. Expansion of compressed air from an intermediate pressure of, say, 120-700 psig. to the nominally 80 psig. pressure of the lower column is mechanically difficult and thermodynamically inefficient. Not only does expansion turbine efficiency decrease with higher inlet-outlet pressure ratios, but in the event a portion of the air liquefies in the expansion turbine there will almost invariably be a further decrease in thermodynamic efficiency.

Attempting to avoid these difficulties by raising the lower column pressure to reduce the pressuredrop across the expansion turbine leads to other problems: thicker column walls, a decrease in relative volatility between oxygen and nitrogen, reduced distillation efficiency, and thus more plates or trays in taller columns. Facing these difficulties, it is not suprising that most, if not all, modern air separation plants are either of the low pressure type or of the high pressure'type. Indeed, the economics of investment and operating cost have generally led to the conclusion that compressors and compression represent the major cost of an air separation plant, and for this reason alone the majority of separation plants are low pressure ones.

Accordingly, a primary object of the invention is to provide an improved air separation process capable of producing one or more liquid products and one or more gaseous products in a variable liquidgas product mix, without requiring the independent liquefiers needed to produce liquids in a conventional low pressure air separation process and without experiencing the mechanical and operational inefficiencies of conventional high pressure processes, which improved process features reduced investment and operating costs as compared to prior art processes.

SUMMARY This invention relates to a process for cryogenic separation of air into at least one liquid product and one gaseous product.

The present invention provides a process for the cryogenic separation of air by fractional distillation into at least one liquid product and one gas product at a high pressure and then at a low pressure wherein the compressed air is cooled by heat exchange with cold streams from the process, comprising the steps of: compressing said air to a head pressure of -700 psig. being higher than said high pressure distillation; selectively adsorbing atmospheric impurities from the compressed air prior to cryogenic cooling so as to provide precleaned air; cooling and work expanding a first minor portion comprising 120% of said precleaned air to a pressure below said high pressure distillation and recovering refrigeration from the resulting cold low pressure gas for the process; cooling and liquefying a second minor portion comprising 120% of said precleaned air to form liquid air and distilling said liquid air with the refrigeration for said liquefying being provided by said cold streams which are thereby partially warmed; partially cooling the major portion of said precleaned air by the partially warmed cold streams to temperature above the temperature of the cooled and liquefied second minor portion, and work expanding the partially cooled major portion to the pressure of said high pressure distillation such that the expanded cold fluid is substantially entirely gas, and flowing at least part of the resulting cold gas to said high pressure distillation; and withdrawing at least one liquid product and at least one gas product from the distillation and discharging such liquid and gas products from the process.

By thus splitting the air to the column into a workexpanded major portion and a liquefied minor portion, work-expansion is effected from a stream maintained at as high a superheat as possible so as to efficiently produce refrigeration in the main turbine. Conversely, by liquefying a minor portion of the incoming air, that is, less than half, advantageously between 1% to about 20% or so, and preferably about 245%, by condensing the portion against returning product streams, a greater portion of refrigeration from these streams is recovered and transferred back into the distillation column. Stated otherwise, by splitting the air feed to the column into a work-expanded major gaseous portion and a liquefied minor portion, the main turbine is enabled to operate at a higher, thermodynamically more efficient, inlet temperature than it could if all the plant air were to pass through the turbine, while the liquefied minor portion is utilized to recover more of the refrigeration from waste and product streams leaving the columns.

Another minor portion of the incoming air, also advantageously in the range of about 1% to about 20%, preferably about 215%, of cooled compressed plant air is separately work-expanded through a second turbine. This provides additional refrigeration for the process. This minor portion may be derived either from a reheated portion of the main turbine discharge or as a side bleed from the feed heat exchanger. In the former case, the system is referred to herein as series cycle, as the two turbines are disposed in series, while in the latter case the cycle is sometimes referred to as parallel cycle as the turbines are in parallel.

In one embodiment of the invention, the discharge from the second turbine is returned to the feed heat exchanger where it provides additional refrigeration, in which event the turbine is referred to an excess air turbine, as this air is not transmitted to the distillation columns of the plant. Alternatively, the discharge is fed to the upper, or low pressure, distillation column for rectification. While specific illustrative embodiments will be discussed where the series cycle is used to provide excess air, and where the parallel cycle is used to provide upper column air, it is evident that the series cycle may provide upper column air and the parallel cycle provide excess air. The choice among these alternatives depends primarily on the expected product mix, that is, the expected requirement for liquid oxygen, nitrogen, and/or argon, and for their gaseous counterparts, as well as purity requirements for each stream.

BRIEF DESCRIPTION OF THE DRAWINGS FIG. 1 schematically depicts a flow sheet of a series cycle embodiment of the invention where the turbines are disposed in series and where the second, or excess air, turbine discharges back through the main feed heat exchanger;

FIG. 2 schematically depicts a flow sheet of a parallel cycle embodiment where the turbines are disposed in parallel, the second or upper column turbine discharging to the low pressure fractional distillation column of the plant; and

FIG. 3 schematically depicts a flow sheet of a compression and pre-adsorption unit designed primarily for the embodiment of FIG. 1 but also useful for the embodiment of FIG. 2.

DESCRIPTION OF THE PREFERRED EMBODIMENTS In the embodiment depicted in FIG. 1, a process is shown for the cryogenic separation of air into liquid nitrogen, liquid oxygen, and gaseous nitrogen. In one specific example, the FIG. 1 embodiment treats 966,000 cubic feet per hour of air at NTP (70F., 14.7 psia.) and produces product in the form of 73,500 CFH equivalent of liquid'nitrogen, 84,000 CF H equivalent of liquid oxygen, and 225,000 CFH of gaseous nitrogen. The liquid oxygen is at least 99.5 mol percent pure, and the product nitrogen stream contains a maximum of 0.004 mol percent oxygen. Based on each 1,000 cubic feet per hour of air at NTP (70F, 14.7 psia.) treated, this process may produce product in the form of 76 CFH equivalent of liquid nitrogen, 87 CFH equivalent of liquid oxygen, and 233 CFH of gaseousnitrogen.

The incoming air is dried and precleaned of atmospheric contaminants such as carbon dioxide and acetylene prior to introduction through conduit 30 for cryogenic cooling. Precleaning may for example be accomplished by molecular sieve type-selective adsorbents such as zeolite A or zeolite X.

Refrigeration to sustain the process and generate the liquid oxygen and nitrogen product requirements is provided by two work expansion turbines, namely the main turbine 11 and the excess air turbine 12. The main turbine 11 expands a major portion of the main plant air stream, admitted by conduit 30, from the plant head pressure of about 615 psia. to the approximately psia; pressure of the lowercolumn 32. The main portion of the exhaust from the main turbine 11 feeds through a conduit 34 to the high pressure column 32 as vapor atclose to the saturation temperature, while the remaining portion not required for rectification is reheated in the feed heat exchanger 14 and provides additional refrigeration by being work expanded through the excess air turbine 12 to a pressure suitable for regeneration of the absorber beds described above. As expansion turbines 11 and 12 are employed in series, the FIG. 1 process may be termined a series cycle.

Refrigeration required to operate the columns 31, 32 and to withdraw the liquid production is initially supplied to the incoming air stream in the conduit 30 by the primary or feed heat exchanger 14. Here the air is refrigerated by several streams, including excess air obtained from the excess air turbine 12 and supplied through. conduit 35, product nitrogen gas supplied by the conduit 36, and a waste predominantly nitrogen stream supplied through the conduit 38.

In the system of FIG. 1, incoming feed air in conduit 30 enters at a rate of 951,000 CFH at NTP, pressure of about 615 psia. and a temperature of about F. in the aforementioned specific design. In the first heat exchanger pass 39, the incoming air exchanges heat with three countercurrent streams, namely the excess air supplied by conduit 35 at a temperature of about 110K., the waste supplied by conduit 38 at a temperature of about 96K., and the product gaseous nitrogen supplied by the conduit 36 at a temperature of about K. When the excess air and waste nitrogen are both released to the atmosphere after warming, conduits 35 and 38 may be joined at the cold end of the first heat exchanger.

Excess air is discharged from the feed heat exchanger 14 by the conduit 29 at a pressure of about 40 psia. The waste stream, admitted to the heat exchanger 14 by conduit 38, is discharged ,to the vent by conduit 40 at an exchanger exhaust pressure of 16 psia. The product nitrogen leaving the liquefying heat exchanger feed by conduit 41 is sent to aproduct compressor, not shown, at an inlet pressure of about 75-80 psia.

Leaving the first heat exchanger pass 39, the feed air is optionally withdrawn from the feed heat exchanger 14 and sent to a forecooler 42 for additional cooling or refrigeration by heat exchange with an extraneous ammonia or halogenated hydrocarbon refrigerant stream. Forecooling is not however essential, but is a convenient way of increasing the liquid oxygen and nitrogen make of the plant. For the relative flows in this illustrative embodiment of FIG. 1, the forecooler 42 is not used.

Upon returning to the feed heat exchanger 14 from the forecooler 42, the plant air stream divides into two portions, with the major portion of the feed air flowing through an exchanger cooling pass 44 and thence by conduit 45 to the main turbine 11. The minor portion flows through an exchanger cooling and liquefying pass 46 where it is liquefied at the cold end of the feed heat exchanger 14 by colder incoming streams to the ex changer 14. Control of the exchanger 14 operation is accomplished by throttling the liquid air to the high pressure column 32 as a function of the main turbine 11 discharge temperature. The function of this minor stream is best deferred for later discussion in connection with operation of the columns 31, 32. Although the liquefied minor portion of precleaned air in conduit 48 is illustrated as being introduced to high pressure column 32, it may alternatively be introduced to low pressure column 31 after suitable throttling.

As noted above, the incoming plant air leaves the feed heat exchanger 14 as two streams, the major portion flowing through conduit 45 and the minor or liquefied portion, e.g. 22% of the feed air leaving the exchanger pass 46 by the conduit 48. This major portion amounting to 739,000 CFH of air atl70K. and 600 psia, is fed to the main turbine 11, while the minor portion in conduit 48 amounts to 212,000 CFH.

The main or lower column turbine 11 work expands the refrigerated air of conduit 45 to provide the low level refrigeration required for liquid production in the plant. By reason of the fact that the main turbine 11 work expands the incoming air before it is sent to the distillation columns 31, 32, the cycle is properly termed a pre-expansion cycle.

The main turbine 11 is a rotary turbine, advantageously a centrifugal rather than an axial turbine. It accepts incoming air at 600 psia and 170K. and expands it to a pressure of 85 psia and a temperature of 102K. In some instances, thermodynamic efficiency may be higher and turbine blade erosion may be avoided by operating turbine 11 entirely in the gas phase, that is, insuring that the expansion does not enter the dome of the temperature enthalpy diagram for air.

The discharge stream 49 leaving the main turbine is then split, with the major portion amounting to 628,000 CFH e.g., 85%, flowing through conduit 34 directly to the bottom of the high pressure column 32. A minor portion, 1 11,000 CFH, is preferably sent by conduit 50 to an additional pass or core 51 in the feed heat exchanger 14, where it is warmed to 148K. at 84 psia before entering the excess air turbine 12. If desired, an auxiliary by-pass 52 can allow a portion of this minor stream to by-pass the core 51 and flow directly to the excess air turbine 12 for trimming the inlet temperature to the turbine 12.

An excess air turbine 12 is arranged in series with the main turbine 11 to effect the second stage of work expansion of a minor portion of the incoming air. This portion, which may range from as little as 5% or less to as much as about 30% or so of the total plant air, provides additional refrigeration to maximize the production of liquid oxygen and liquid nitrogen in the plant.

As noted earlier, compressed air fed to the excess air turbine 12 is composed of a portion of the discharge from the main turbine 11, warmed from 102K. in the pass 51 of the feed heat exchanger 14, and optionally a small amount bled directly from the main turbine outlet to the valved by-pass conduit 52. Both streams are at pressures of about 8485 psia, and are work expanded in the excess air turbine 12 to a discharge pressure of about 40 psia, and a corresponding discharge temperature of about 110K.

In this series cycle, it has been found desirable to uti lize the refrigeration produced by the excess air turbine 12 solely as a refrigerant to supply refrigeration to the incoming air in the feed heat exchanger 14. No portion of the discharge from the excess air turbine 12 is fed to either distillation column 31 or 32.

Excess air leaving the main turbine 12 by conduit 35 discharges through the core or pass 52 of the feed heat exchanger and conduit 29.

The low pressure column 31 and the high pressure column 32 operate in substantially conventional manner to effect the low temperature fractional distillation of air into its principal components, namely nitrogen and oxygen. The incoming air'is first fed to the high pressure column 32, operating at a nominal pressure of psia, and then most of the separated components are fed to the low pressure column 31, operating at a nominal pressure of 25 psia. These pressures may in fact be varied over considerable limits, provided only that the temperature at the top of the high pressure column 32 is higher than that of the botton of the low pressure column 31 in order that vapors leaving the high pressure column 32 can serve as reboiler heat medium for the low pressure column.31., In the specific example herein described, the low pressure column 31 has 21 trays and the high pressure column 32 has 45 such trays. The overhead from the high pressure column 32 produces the liquid nitrogen product, while the bottoms of the low pressure column 31 is the liquid oxygen product of the process.

Feed to the high pressure column 32 is composed of two streams obtained by splitting the incoming air as described earlier, The major stream, entering by conduit 34, is a substantially entirely gas stream taken directly from the main turbine 11 at a rate of 628,000 CFI-I. The minor-stream, entering by the conduit 48 and a throttle valve 54, is compressed air at least partially liquefied in the liquefying core or pass 46 of the feed heat exchanger 14. Alternatively, the throttled minor stream may be introduced to low pressure column 31 instead of high pressure column 32. If the throttled stream is a liquid-gas mixture it may be separated with the liquid joining the kettle transfer liquid in conduit 65 and the gas passed to the high pressure column 32 for separation.

The use of a split air feed to the high pressure column 32 (and alternatively in part to the low pressure column 31) constitutes a significant process refinement of the invention. The ability of a coproducts plant to produce substantial quantities of liquid depends on the effective generation and utilization of refrigeration, and the process of the present invention are unique in obtaining substantial quantities of liquid product in plants operating at relatively low head pressures.

This efficiency is realized by permitting air to expand at the main turbine from as high a temperature as possible consistent with the recovery of refrigeration. By splitting the air prior to the main turbine 11 and condensing the minor portion against returning waste and product streams in the feed heat exchanger 14 (or in-a product superheater in the parallel cycle to be described presently), this minor portion is liquefied and thereby recovers refrigeration normally lost in the returning, or discharging, streams. The recovered refrigeration is transferred back into the column 32 (and/or column 31) with the liquefied air of conduit 48, and permits the net production of liquid oxygen and liquid nitrogen without the necessity of a separate liquefier.

Moreover, by liquefying and cooling the minor portion of incoming compressed air, the inlet temperature to the main turbine 11 may be maintained at a higher value than would be otherwise required for the provision of a liquid-containing feed air to the high pressure column 32 (and/or column 31). Thus, the main expansion turbine 11 is enabled to operate in the most efficient manner by utilizing expansion from as high a temperature as possible to generate refrigeration.

In the high pressure column 32, vapors ascend which are progressively richer in nitrogen, while contacting a descending reflux stream of liquid nitrogen which becomes progressively enriched with oxygen as it descends through the column 32. ,Vapors leaving the top of the column 32 by an overhead line 55 are thus predominantly nitrogen, and are in part cycled through the reboiler 56 and in part discharged by the conduit 36 as product gaseous nitrogen, ultimately to the product compressor, not shown, receiving nitrogen by conduit 41.

Vapors from the high pressure column 32 entering the low pressure column 31 reboiler 56 condense in the reboiler as so-called shelf nitrogen. A portion of the shelf nitrogen is returned to the high pressure column 32 by conduit 58 as reflux to the column, while a second portion is withdrawn by conduit 59 at a temperature 95K. as product liquid nitrogen. This product is passed to superheater 60 which supercools the liquid to 85K., after which it is sent to a separator 61 by conduit 62 and throttle valve 64 for further flash cooling. The final liquid nitrogen product is withdrawn as a bottoms from the separator 61 at a rate of 73,500 CFH, pressure of psia, and a temperature of 82K. It contains about 0.0004% oxygen.

Meanwhile, the bottoms product, or kettle liquid, of the high pressure column 32 is withdrawn from the column by bottoms conduit 65 and conducted to the pass 66 in the superheater 60. There it is cooled and then passed through a throttle valve 68 in conduit 69 to pro vide liquid feed to the top of the low pressure column 31. This feed is composed of about 32.6 mol percent oxygen. If the liquefied minor portion of precleaned air in conduit 48 is to be introduced to the low pressure instead of the high pressure column, it may be joined with the kettle liquid in conduit 65 upstream cooling pass 66 or alternatively cooled in a separate pass of superheater 60.

The low pressure column 31 effects separation of the feed into a waste gaseous overhead product withdrawn by overhead line 70, and a 99.5 mol percent liquid oxygen product in line 57 from the bottoms. The waste overhead of line 70 combines with flashed liquid nitrogen from the separator 61 in overhead line 71 and the combined waste gases, now at a temperture of about 84K., are conducted by a cooling pass 72 in the product superheater 60. This waste stream isthen sent by conduit 38 to the main feed heat exchanger 14 for additional refrigeration recovery. Since the flashed gas in overhead line 71 is high purity gaseous nitrogen it may be recovered as product. In this event it may be combined with high purity gaseous nitrogen in conduit 36 provided that the latter is throttled to the pressure of the flash gas in 71 before withdrawal. In this event, flash gas 71 would not be combined with overhead waste gas 70.

If it is desired to increase the liquid oxygen production at the expense of lower nitrogen production, the shelf nitrogen liquid and the kettle liquid should be transferred to the low pressure column in the conventional manner. That is, the kettle liquid in conduit 69 is introduced at an intermediate level and a portion of the shelf liquid in conduit 59 is introduced as reflux at the top of column 31.

It is therefore apparent that the series cycle produces a substantial amount of both liquid nitrogen (76 CFH) and of liquid oxygen (87 CFI-I), together with gaseous nitrogen (233 CFI-I), per 1,000 CFI-I air feed. The liquid product is unusually high for a plant operating at so low a head pressure and utilizing no extraneous refrigeration. As a result, a coproduct cycle is provided which is capable of producing one or more liquid products together with one or more gaseous products. Further, the product streams can be varied over relatively wide limits to thereby increase the production of either liquid as customers demand changes.

Summarizing the FIG. 1 embodiment, the compressed air at -700 psig. head pressure is cleaned of atmospheric impurities prior to cryogenic cooling by selective adsorption of same. A first minor portion comprising 120% of this precleaned air is cooled and work expanded to pressure below the high pressure distillation as cold low pressure air. The latter is heat exchanged with the precleaned air and the so-warmed low pressure air is released from the process as excess air. A second minor portion comprising 120% of the precleaned air is cooled and liquefied to form liquid air which is then distilled, the refrigeration for such cooling and liquefying being provided by the cold streams which are thereby partially warmed. The major portion of the precleaned air is partially cooled by the partially warmed cold streams to temperature above the temperature of the cooled and liquefied second minor portion, work expanded to the pressure of the high pressure distillation for partial separation therein, and a smaller part of the cold gas is heat exchanged with the second minor portion of precleaned air for said cooling and liquefying thereby reheating said smaller part. The soreheated smaller part is employed as the first minor portion of'precleaned air to be work expanded.

Liquid oxygen and liquid nitrogen products are withdrawn from the distillation as separate streams and these liquid products are discharged from the process. Gaseous nitrogen product is also discharged from the distillation and heat exchanged with the precleaned air as another of the cold streams for the air cooling, the so-warmed gaseous nitrogen product being recovered from the process. Gaseous nitrogen waste is also discharged from the distillation and heat exchanged with the precleaned air as still another of the cold streams for the air cooling and the so-warmed gaseous nitrogen waste is released from the process.

In the embodiment depicted in FIG. 2 a process is shown for the cryogenic separation of air to provide maximum liquid oxygen, together with gaseous nitrogen and oxygen. The process is also adapted, as also will be described, for the manufacture of liquid nitrogen. Approximately 7% of the incoming air is recovered as liquid product.

The process of FIG. 2, hereafter sometimes referred to as the parallel cycle, utilizes two expansion turbines, namely the main turbine 111 and the upper column turbine 112, disposed in parallel. Process air to the upper column turbine 112 is withdrawn as a side stream from the heat exchanger 113, 114, rather than as a portion of the discharge from the main turbine as described in FIG. 1. Also in contrast to the embodiment of FIG. 1, the upper column turbine 112 dischrges to the upper, or low pressure, column 131, rather than back through the feed exchanger, and thus provides additional air for rectification in the columns.

In one example designed to treat 2,600,000 CFH air at NTP, the FIG. 2 process will produce the equivalent of 180,000 CFH liquid oxygen, 324,000 CFH of 99.5% gaseous oxygen, and 524,000 CFH gaseous nitrogen. Based on each 1,000 cubic feet per hour of air at NTP (70F, 14.7 psia) treated, this example of the FIG. 2 process will produce 90 CFH equivalent of liquid oxygen, 125 CFH of 99.5% gaseous oxygen, and 201 CFI-I (all at NTP) of gaseous nitrogen containing less than ten parts per million oxygen.

To afford a convenient basis for comparing the parallel cycle of FIG. 2 with the series cycle of FIG. 1, generally equivalent components of FIG. 2 have been designated with identifying numerals that are 100 units higher than the corresponding element of FIG. 1.

The incoming air is provided to the system of FIG. 2 at a pressure of 287 psia., and a temperature of 279.4K. Preadsorption reduces the carbon dioxide to a maximum level of about 0.25 ppm., desirably about 0.1 ppm. or less. Water is removed in molecular sieve absorbers to a dew point of below 50F., and more usually below 100F.

Refrigeration for the parallel cycle of FIG. 2 is provided by two work expansion turbines, the main turbine 111 and the upper column turbine 112. The main tur bine 111 expands a major portion of the plant air stream from the plant head pressure of about 287 psia. to the pressure of the lower, or high pressure, column 132, in this case about 97 psia. The upper column turbine 112 expands a minor portion of the incoming plant air from the 287 psia. head pressure to the psia. pressure of the low pressure column.

Compressed air entering the process of FIG. 2 is refrigerated by indirect countercurrent heat exchange with cold streams leaving the process in a pair of feed heat exchangers 113, 114.

Incoming air at 287 psia. and 279.4K. is admitted to the process by conduit 130 and split into two conduits, 181 and 182, prior to the respective heat exchangers 113, 114. Flow rates through the conduits 181, 182 are proportioned so as to balance the flows and heat loads through the two exchangers.

Each feed exchanger 1 13, 1 14 is provided with three passages or flow paths. The feed air passages 184, 185 in the respective exchangers 113, 114 are provided with side bleeds 186, 188, respectively, so that a portion of the air through the passages 184, 185 is drawn off at an intermediate point of the passages and does not receive full refrigeration. These bleeds are positioned at intermediate locations along the heat exchanger to achieve desired bleed air temperatures for varying air input rates and conditions and differing product mixes.

Both heat exchangers 113, 114 exchange refrigeration with gaseous oxygen product of the process. Gaseous oxygen, approximately 99.5% pure, is withdrawn from the low pressure column 131 by conduit 189, warmed in liquefier 190 (to be described further), and then split into two streams admitted respectively by the conduits 191, 192 to the feed heat exchangers 113, l 14 for exchange with the incoming plant air. Gaseous oxy gen product is discharged from the exchangers 113, l 14 as a stream in conduit 194.

In addition to exchanging refrigeration vw'th gaseous oxygen product, that portion of incoming air admitted to the feed exchanger 113 exchanges refrigeration with product gaseous nitrogen in the exchanger 113. The nitrogen is admitted by conduit 136 to the exchanger 113 and withdrawn by conduit 141. Similarly, the half of the incoming air admitted to the feed heat exchanger 114 by conduit 185 exchanges refrigeration with waste product, predominantly nitrogen, admitted to the exchanger 114 by the conduit 138.

Plant air leaving the feed heat exchangers 113, 114 is divided into three streams. First, the major portion, at a pressure of 282 psia. and a temperature of 139.3K., is directed by conduit to the main work expansion turbine 111. A first minor portion at the same pressure and temperature is directed by conduit 148 to a liquefier pass 146 in the product superheater 160, throttled in valve 148a and then introduced as a liquid feed stream to the high pressure column 132. A second minor portion, withdrawn as a side bleed 186, 188 at apressure of 282 psia. and a temperature of K. is sent by conduit 152 to the upper column turbine 112 and thereafter serves as an additional feed to the low pressure column 131. In this specific embodiment, low pressure column 131 has 57 sieve type trays and high pressure column 132 has forty-one such trays.

The main, or lower column turbine 111, work expands the major portion of the incoming plant air to provide low level refrigeration for liquid production in the plant. As the main turbine 111 work expands the incoming air before it is sent to the fractionation or distillation columns 131, 132, the parallel cycle is also a pre-expansion type liquefaction system.

The main turbine 111 is a rotary turbine advantageously of the centrifugal type, which accept incoming air by conduit 145 from the feed heat exchangers 113, 114 at a pressure of 280 psia. and a temperature of 139.3I(. and, at an efficiency of about 82%, expands it to a pressure of about 100 psia. The expanded air temperature is about 105.8K., which is about 3.8 above the saturation temperature of air at 100 psia.

Air expanded through the main turbine 111 at flow rate of 1,975,000 CF H into conduit 149 is split into two streams, both being fed to the high pressure column 132. One stream, constituting the major part, e.g., 99.4% or 1,964,000 CFH is fed directly to the bottom or kettle of the higher pressure column 132 by conduit 134. The minor part comprising 11,000 CFH is directed'to a liquefier pass 151 in the liquefier where it is liquefied by indirect countercurrent heat exchange with oxygen product from the bottom of the low pressure column 131 conducted to the liquefier by conduit 189. Alternatively, the minor part may be bypassed around turbine 111 directly to conduit 150. This minor part of the main turbine 111 discharge fed to the liquefier pass 151 exits at a temperature of about 100.5K. and is conducted by the conduit 150 to the lower column 132. This stream joins the liquefied air withdrawn from the liquefier pass 146 by a conduit 148 at flow rate of about 268,000 CFH, a pressure of about 97 psia., and a temperature of about 108K. The combined stream is admitted to the lower column 132 at a pressure of about 97 psia., and a temperature of about 100.8K., and is admitted to the conduit as a liquid feed to the high pressure column 132. Alternatively, at least part of the liquefied air streams in conduits 148 and 150 may be introduced to the low pressure column 131 instead of high pressure column 132, preferably after subcooling in product superheater 160.

The upper column turbine 112 is in parallel with the main turbine 1 1 1 and work-expands a minor portion of the refrigerated incoming air to provide additional refrigeration to maximize the production of liquid products in the plant. This minor portion, which may range from as little as or less to as much as about or more of the total plant air admitted to the conduit 130, is fed to the upper column turbine 1 12 at a higher temperature than the feed air to the main turbine 111 to produce the greatest amount of refrigeration possible. When the temperature at the expansion turbine 112 inlet is at the highest level possible consistent with the recovery of refrigeration at the desired temperature level, the turbine produces the maximum refrigeration.

Incoming air at 357,000 CFH to the upper column turbine 112 is at an inlet pressure of 282 psia. and an inlet temperature of 180K. It is work-expanded in the upper column turbine 112 to a pressure of about psia. and discharged directly by the conduit 135 to the low pressure column 131 as a gaseous feed above the twenty-fourth tray. With an efficiency of 74% for the upper column 112, the discharge temperature is about 109K; the saturation temperature at about 24.7 psia. is 87K.

The upper column turbine 1 12 provides a useful control over the product mix from the parallel cycle. To maximize liquid production but at a concurrent diminution of plant throughput, more secondary air is admitted to the upper column turbine 112. Alternatively, to minimize liquid production, less air is admitted. In the event no liquid product is desired and the parallel cycle is used only for gaseous oxygen and nitrogen production, the upper column turbine 112 may be taken out of service and no air sent by the conduit 135 to the upper or low pressure column 131.

Feed to the high pressure column 132 is composed of three streams, as described earlier. The major stream, entering the reboiler by conduit 134, is an exclusive gas stream of 1,964,000 CFH obtained directly from the discharge of the main air turbine 111 and is at a temperature of about 105.8K. The minor stream (14.2% of major stream flow rate) entering the column 132 is at a flow rate of 279,000 CFH and a pressure and temperature of 97 psia. and 197K., respectively, and is composed of the two minor portions of the refrigerated and liquefied plant air described earlier. In particular, it is composed of the refrigerated air withdrawn from the passes 184, 185 of the feed heat exchangers 113, 114 by the conduit 148 (13.6% of major stream flow rate) liquefied in the liquefier pass 146 of the superheater 160, and admitted to the column 132 by conduit 148 at a flow rate of 268,000 CFH, and of a minor portion of the discharge from the main air turbine 111 taken by conduit 150 through the liquefier pass 151 of the liquefier 190.

When, in the case being described, a liquid oxygen product is being withdrawn from the process, an additional stream of shelf nitrogen from the high pressure column 132 is withdrawn from the reboiler 156 of the low pressure column 131 and conducted by conduit 198 to the liquid oxygen subcooler 199 which cools the liquid oxygen product below its boiling point. This shelf stream of conduit 198, constituting about 12,000 CFH, is withdrawn from the subcooler 199 and fed by conduit 200 to the waste gas pass or core 172 in the product superheater 160, from whence it is conducted to the feed preheater 114 and thereafter disposed of as waste. Since gaseous nitrogen in conduit 200 is high purity prior to joining the waste gas. it may alternatively be combined with the high purity nitrogen in conduit 170 and warmed in heat exchanger passage 173.

The upper or low pressure column serves to produce high purity oxygen and is fed with three streams: 357,000 CPI-I air from the upper column turbine 112 at 25 psia. and 109K., a predominantly (maximum 13% vapor) liquid feed from the kettle of the high pressure column 132 supplied at a rate of 1,305,000 CFH at about 9614. and shelf nitrogen by a core 201 at a rate of 892,300 CFI-I. In the aforementioned 57 tray lower pressure column, air from the turbine 1 12 enters below the 24th tray; kettle liquid enters above the 34th tray; and shelf nitrogen enters at the top above the 57th tray.

The low pressure column 131 concentrates nitrogen at its upper section and oxygen at its kettle or lower section. Nitrogen product is withdrawn from the top vapor line 170 as the coldest stream in the plant at a rate of 924,000 CFH, a pressure of about 19.2 psia., and a temperature of 79.7K. Its oxygen content is less than about 10 ppm. This stream is fed to the product superheater 160 where it is successively passed in countercurrent heat exchange relationship with three streams: (a) to subcool shelf nitrogen in the pass 201 before it enters the low pressure column 131, (b) to subcool kettle liquid in the pass 166 before the liquid enters as a feed to the low pressure column, and (c) to liquefy the minor portion of the incoming plant air in the liquefier pass 146 before this is fed to the high pressure column 132. Additional refrigeration for the product superheater 160 is afforded by withdrawing a waste stream by conduit 202 from just below the top deck of the low pressure column 131 at 1,559,300 CFH and combining this with the waste stream in conduit 200. The total stream of 1,572,000 CFH at a temperature of about 81K., contains about 2.74% oxygen, and is heated in the superheater 160 to about 122K. and thereafter in the main feed preheater 114 to about 278K., after which it is vented to waste.

The liquid oxygen product (see conduit 157 is available at the bottom or reboiler of the low pressure column 131. Liquid in the reboiler is pumped at a rate of 300,000 CFH through a bed of silica gel adsorbent 204 and then split, with one portion returning by conduit 205 to the low pressure column 131 while a second portion is transmitted to the liquid oxygen subcooler 199 where it becomes the subcooled liquid product of the process. Silica gel serves as a secondary protection to remove traces of carbon dioxide which may have escaped the initial molecular sieve treatment prior to compression.

Under the conditions just described for the plant design based on the parallel cycle of FIG. 2, a plant could produce 180,000 CFl-I of 99.5% pure oxygen at a pressure of 25 psia. and subcooled to K. It also yields 324,000 CFH of gaseous oxygen through conduit 194 at the same purity (at a pressure of 23 psia. and a temperature of 273.3K), and a gaseous nitrogen product containing at most 10 ppm. oxygen via conduit 141 at a flow of 124,000 CFl-l (and at a pressure and temperature of 14.8 psia. and 278.3K., respectively). Liquid nitrogen, if desired, may be taken from the shelf.

Although the process of FIG. 2, as described, produces about 7% of the air as liquid oxygen, variations of the process conditions may be utilized to vary the product mix. Liquid oxygen make may be increased at however a sacrifice in plant through capacity, by increasing the excess air flow through the upper column turbine,112. Conversely, the liquid oxygen make may be decreased to about 155,000 CFH by completely eliminating the flow through the turbine 112.-This latter optional mode is desirable when the highest possible oxygen recovery is desired.

For the manufacture of gas only in the parallel cycle, flow through the main turbine 111 is terminated and flow through the upper column turbine 112 is increased. Based on each 1,000 CFI-I of air feed (or total of 2,440,000 CHF air) and a flow through the upper column 1 12 of 150 CFI-I (or total of 366,000 CFH), the parallel cycle of FIG. 2 is capable of producing 204 CFI-I (or total of 500,000 CFH) of gaseous oxygen product and 199 CFI-I (or total of 485,000 CFH) of gaseous nitrogen at the specified purities. In this case, refrigeration to sustain the process is'provided by the upper column turbine 112, and a flow of 850 CFI-I (or total of 2,071,000 CFI-I) is merely throttled across a valved line by-passing the main turbine 111.

Summarizing the FIG. 2 embodiment, a process is provided for cryogenic separation of air into liquid oxygen product and gaseous oxygen and nitrogen products by fractional distillation at high pressure and then at low pressure wherein compressed air is cooled by heat exchange with cold streams from the process. A first minor portion comprising 120% of the precleaned air at the head pressure is cooled by a cold stream to form liquid air which is distilled for separation. A second minor portion comprising 120% of the precleaned air is partially cooled by heat exchange with the cold streams, work expanded to the pressure of the low pressure distillation and passed thereto for separation. The major portion of the precleaned'air is cooled to lower temperature than the partially cooling and work expanded to the pressure of the high pressure such that the expanded cold fluid is substantially entirely gas. A major part of the work expanded major portion is passed to the high pressure distillation for partial separation therein, and a minor part is heat exchanged with cold gaseous oxygen product from the low pressure dis tillation thereby liquefying the minor part and partially warming the gaseous oxygen.

Liquid oxygen is withdrawn from the distillation and discharged from the process as liquid product. Gaseous nitrogen is also discharged from thedistillation and heat exchanged with the first minor portion of pre cleaned air as the cold stream for liquefying thereof. The partially warmed gaseous nitrogen is heat exchanged with the precleaned air as another of the cold streams, and at least part of the so-warmed gaseous nitrogen is recovered as product.

Referring to FIG. 3, air is taken into the system in a filter house 15 and at 85F. may for example be 65% saturated with water vapor and contain 300 ppm. carbon dioxide. From the filter house 15, the air is compressed by a four-stage centrifugal type of rotary compressor 16, equipped with interstage and postcompression coolers, to a discharge pressure of 121 psia. and a discharge temperature of 90F. Air from the compressor 16 is then conducted via conduit 18 to a centrifugal booster compressor 19, which is of twostage design and is equipped with interstage and poststage water coolers. Here the air is compressed to a pressure of 467 psia. and a discharge temperature of 90F. Both the compressors 16 and 19 are driven by electric motors or equivalent prime movers.

Air from the booster compressor 19 is conducted via conduit 20 to a third rotary compressor 21, in this case a rotary compressor 21 driven by the main turbine 11 (FIG. 1). The compressor 21 increases the air pressure to 615 psia. and, after water cooling to restore the F. temperature, transmits the air via conduit 22 to a water separator pot 24. Most of the initial water is withdrawn at the separator pot 24 and periodically sewered.

Air at the separator pot 24 normally contains several impurities that must be excluded from the cryogenic fractionation. Water vapor, present in an amount corresponding to the vapor pressure of water at the temperature of the separator pot 24, will freeze out on heat exchanger surfaces unless effectively excluded. Carbon dioxide, usually present in air at a concentration of about 300500 ppm., must be removed for the same reason. Hydrocarbons, the presence of which is unavoidable as a result of air pollution, offer a more serious problem; in the presence of high purity oxygen, concentrations of hydrocarbon can develop and form an explosive mixture.

In keeping with an important feature of the process, these several impurities are reduced to an inocuous concentration level by passing the plant air from the separator 24 through a molecular sieve bed. Molecular sieves, primarily Type 13X or 10X (Milton U.S. Pat. No. 2,882,244), but also Type 5A or 4A (Milton US. Pat. No. 2,882,243) where a lower capacity for carbon dioxide can be tolerated, have the ability of selectively removing the more deleterious impurities from air. Molecular sieves are crystalline aluminosilicates which comprise basically a flireedimensional framework of SiO., and AlO, tetrahedra cross linked by the sharing of oxygen atoms. The sieves contain, or at least originally contained, exchangeable or removable cations in the crystal. Molecular sieves are further characterized by having interconnected sorption areas on the inside of the framework which are accessible through substantially uniform pore openings of molecular dimensions.

Carbon dioxide is readily removed from the air by Type 13X molecular sieves to a residual average level of about 0.1 ppm.; a level of 0.25 ppm. is the design maximum, and the plant is equipped with an automatic detector, not shown, which shuts down the plant should the carbon dioxide content exceed 5 ppm. Similarly, water is removed to a dew point of below 50F., and more usually below about F. Acetylene and higher hydrocarbons are undetectable in the effluent from a properly operated 13X molecular sieve bed.

As shown in FIG. 3, the process employs two adsorbers 25, 26 where water, CO acetylene, and the higher hydrocarbons are removed. These adsorbers are operated alternately; one adsorber is purifying feed air while the other is being regenerated by passing a regenerative gas, in this case excess air (from a supply 29 in FIG. 3) at about 600F., downward through the adsorber.

The regeneration sequence consists of five steps: depressurization of the adsorber 25 or 26, heating, cooling, repressurization, and optional blending. For the depressurization step, the adsorber is depressured to atmospheric pressure and the gas evolved is vented. A heater 28 in the excess air line 29 is fired up, and excess air is fed through the heater 28 and heated to a temperature of about 600F. It is introduced to the adsorber 25 or 26 to heat the molecular sieve to a temperature of about 575F. so as to desorb the water, carbon dioxide, etc. removed from the incoming plant air. Heating is continued with sufficient gas flow to carry these contaminants out of the bed.

After the bed is fully regenerated, it is cooled to about 100F. by a flow of excess air from conduit 29 (FIG. 3) at 90F. The excess air passes countercurrent to the direction of feed air flow, and is vented to the atmosphere. To repressure an adsorber before returning it onstream, process air is slowly admitted to the adsorber until its pressure corresponds to the pressure of the separator pot 24, namely approximately 615 psia. Due to the exothermic heat of adsorption, the bed warms to about 120F., which may in some cases be too hot to allow the regenerated adsorber to be brought onstream directly. Therefore, the adsorber is gradually cooled to adsorption temperature by blending, or placing it on parallel feed flow, with the onstream adsorber.

When this regeneration sequence is completed, the regenerated adsorber is switched to the onstream position and the previously onstream adsorber is passed through the regeneration cycle as described.

Plant air from the adsorber or pre-purifier or 26 is then conducted via a conduit 30 and a filter 31 to the cryogenic section of the plant.

The use of molecular sieve adsorbents to remove water, carbon dioxide, and hydrocarbons contributes many advantages to the cryogenic air separation process. In comparison with the earlier use of carbon dioxide scrubbers and filter systems, no carbon dioxide or water is allowed to enter the cold box enclosing the feed heat exchanger 14, the low pressure column 31, the high pressure column 32 (all in FIG. 1), and associated low temperature auxiliaries. As a result plant thaws, where the entire plant is thawed to remove ice and frozen carbon dioxide snow, need be carried out at significantly less frequent intervals. Also, the cold box construction itself is simplified in that there is not a scrubber, scrubber liquefier, or scrubber transfer filter and related valving. In addition, molecular sieves remove a substantial fraction of all hydrocarbons except methane, and entirely remove all hazardous, unstable hydrocarbons. From an operational standpoint, operating and maintenance problems required by scrubbers are eliminated; from experience these have included the plugging of scrubber instrument lines, inadvertent scrubber flooding and resulting carbon dioxide contamination of the columns 31, 32, accidental freeze-up of cold leg heat exchangers, rupture of filter elements, plugging of scrubber transfer lines, plugging of the scrubber itself, and in the extreme, plant explosions.

F rom a process standpoint, the use of molecular sieve adsorbents has had an even more significant impact. Prior to the availability of molecular sieves in about 1955, it had been customary to use two complete sets of heat exchangers (e.g., as in FIG. 2) for cryogenic cooling of the incoming air. The best existing adsorbents and carbon dioxide scrubbers were incapable of reliabiy removing water vapor and carbon dioxide to a level sufficient to prevent ice and carbon dioxide snow from depositing in one set of exchangers. This necessitated periodic switching to an alternate set of exchangers while the plugged exchangers were being purged, or derimed.

The art had used two different types of exchangers, regenerators and countercurrent reversing heat exchangers. Both these heat exchange devices operate by periodically reversing the flows of gas through passages wherein ice and carbon dioxide snow are formed and are subsequently removed by subliming the deposited ice and CO snow. Neither system was particularly attractive, and it was necessary to reduce the cold end temperature difference either by the use of a side or central bleed and an associated gel trap, or by the use of a cold end recycle stream (US. Pat. No. 2,460,859). In either case, the process flow was complicated, the reversing equipment was expensive and troublesome and experienced metal fatigue and chloride corrosion, and purge gas and blowdown losses were high. The purge gas loss, as noted earlier, can readily cause the loss of a substantial fraction of the plant nitrogen product. Although the blowdown losses, that is, losses of compressed gas due to the need for repressurizing and depressurizing the exchanger are only about 2% in a plant operating at about psig, they are 4% in a 200 psig plant and 12% in a 600 psig plant. Concurrently, these blowdown losses impose a constraint on the thermodynamics of the cryogenic system as they consume energy without any concurrent benefit in product recovery of quality.

Thus, the use of molecular sieves in the adsorber is both thermodynamically and economically desirable in a plant with the system of the invention. Although it may be eliminated, the practical penalty in terms of in vestment and operating costs dictates the employment of molecular sieve pre-adsorption treatment.

In summary, this invention combines certain process steps in an unexpectedly advantageous manner so as to provide liquid and gas products at lower plant investment and power costs than heretofore. By precleaning the compressed air before cryogenic cooling, blowdown losses are drastically reduced and the cold end temperature difference in the feed heat exchanger need not be closed in the conventional manner of regenerators and countercurrent reversing heat exchangers wherein the air impurities are deposited at cryogenic temperature. With a larger cold end A T the heat transfer rate per square foot of heat exchanger surface is increased and the efficiency of heat transfer is significantly increased. Work expansion of the partially cooled major portion of precleaned air from the feed heat exchanger at relatively warm temperature provides more low temperature refrigeration than if this stream were work expanded from the lower temperature usually associated with the cold end of reversing heat exchangers and regenerators. This additional refrigeration tends to counterbalance the latent and sensible refrigeration lost in the withdrawn liquid product. Moreover, the cooling of a second minor portion of the precleaned air to lower temperature than the partial cooling of the major portion (and sufficient for liquefaction) permits maximum refrigeration recovery from the cold process streams in the distillation section where the liquid air is introduced. This second minor portion cold process stream heat transfer in the feed exchanger in turn thermodynamically benefits the major portion cooling in the same exchanger because the process streams are thereby partially warmed and unable to cool the major portion below the partially cooled level desired as the work expander inlet temperature. As previously indicated, precleaning of the feed air prior to cryogenic cooling in the feed heat exchanger eliminates the need for cooling this air to the impurity dew point from which work expansion would be thermodynamically inefficient.

It should also benoted that absent feed air precleaning at ambient temperature, adsorptive removal of impurities from the major portion of air (only partially cooled according to this invention) would not be as thermodynamically efficient as the prior artreg'enerated or countercurrent heat exchanger and cold end adsorption system. This is because the capacity of adsorbents is substantially less at the partially cooled level of the major portion in conduits 45 and 145 compared with the colder temperatures associated with the closed A T cold end condition required by the prior art. Because of this lower adsorptive capacity for air impurities, adsorbers used in conduit 45 or l45 with the system of this invention would be nearly as large as adsorbers 25, 26 (FIG. 3). However in contrast to the latter which operate at ambient temperature and do not entail refrigeration loss during the regeneration sequence, such adsorbers operating at the partially cooled level of the air major portion dischargedfrom feed exchanger 14 or 1 14 would necessarily be warmed from about 170K. to about 600F. for regeneration and recooled to 170F. for reuse a substantial refrigeration loss.

Although certain embodiments have been described in detail, it will be appreciated that other embodiments are contemplated along with modifications of the disclosed features, as being within the scope of the invention.

What is claimed is:

l. A process for the cryogenic separation of air into at least one liquid product and at least one gas product by fractional distillation at high pressure and then at low pressure wherein compressed air is cooled by heat exchange with cold streams from the process comprising the steps of: compressing said air to a head pressure of 120-700 psig, being higher than said high pressure distillation; selectively adsorbing carbon dioxide and acetylene atmospheric impurities from said compressed air prior to cryogenic cooling so as to provide precleaned air; cooling and work expanding a first minor portion comprising 1-20% of said precleaned air to a pressure below said high pressure distillation and recovering refrigeration from the resulting cold low pressure gas for the process; cooling and liquefying a second minor portion comprising 120% of said precleaned air to form liquid air and distilling said liquid air, with the refrigeration for said liquefying being provided by said cold streams from the process which are thereby partially warmed; partially cooling the major portion of said precleaned air by the cold streams which have been partially warmed, to temperature above the temperature of the cooled and liquefied second minor portion, and work expanding the partially cooled major portion to the pressure of said high pressure distillation such that the expanded cold fluid is substantially entirely gas, and flowing at least part of the resulting cold gas to said high pressure distillation; and withdrawing at least one liquid product and at least one gas product from the distillation and discharging such liquid and gas products from the process.

2. A process according to claim 1, wherein said first minor portion of said precleaned air to be work expanded is a diverted part of the work expanded major portion of said precleaned air which is reheated prior to said work expanding to. pressure below said high pressure distillation.

3. A process according to claim 1 wherein said first minor portion of said precleaned air to be work expanded is cooled to an intermediate temperature, said major portion of said precleaned air to be work expanded is cooled to a lower-than intermediate temperature, and said minor and major portions are thereafter work expanded in parallel flow relation.

4. A process according to claim 1, wherein the work expanded first minor portion is thereafter heat exchanged with the precleaned air for cooling thereof as one of said cold streams.

5. A process according to claim 1, wherein said first minor portion is work expanded to. the pressure of the low pressure distillation and thereafter passed to said low pressure distillation.

6. A process according to claim 1 wherein 215% of the precleaned air is cooled and liquefied as said second minor portion.

7. A process for the cryogenic separation of air into liquid oxygen and nitrogen products and gaseous nitrogen product by fractional distillation at high pressure and then at low pressure wherein compressed air is cooled by heat exchange with cold streams from the process comprising the steps of: compressing said air to head pressure of -700 psig., being higher than said high pressure distillation; selectively adsorbing carbon dioxide and acetylene atmospheric impurities from said compressed air prior to cryogenic cooling so as to provide precleaned air; cooling and work expanding a first minor portion comprising l20% of said precleaned air to pressure below said high pressure distillation as cold low pressure air and heat exchanging same with said precleaned air as one of said cold streams, and releasing the so-warmed low pressure air from the process as excess air; cooling and liquefying a second minor portion comprising 120% of said precleaned air to form liquid air and distilling said liquid air, with the refrigeration for said cooling and liquefying being provided by said cold streams from the process which are thereby partially warmed; partially cooling the major portion of said precleaned air by the cold streams which have been partially warmed, to temperature above the temperature of the cooled and liquefied second minor portion, and work expanding the partially cooled major portion to the pressure of said high pressure distillation such that the expanded cold air is substantially entirely cold gas; introducing a larger part of said cold gas to said high pressure distillation for partial separation threin; heat exchanging a smaller part of said cold gas with said second minor portion as one of said cold streams for said cooling and liquefying thereby reheating said smaller part, and employing the reheated smaller part as said first minor portion of precleaned air to be work expanded; withdrawing liquid oxygen and liquid nitrogen. products from the distillation as separate streams and discharging such liquid products from the process; discharging gaseous nitrogen product from the distillation and heat exchanging same with said precleaned air as another of said cold streams for said cooling thereof, and recovering the so-warmed gaseous nitrogen product; and discharging gaseous nitrogen waste from the distillation and heat exchanging same with said precleaned air as still another of said cold streams for said cooling thereof, and releasing the sowarrned gaseous nitrogen waste from the process. 

1. A PROCESS FOR THE CRYOGENIC SEPARATION OF AIR INTO AT LEAST ONE LIQUID PRODUCT AND AT LEAST ONE GAS PRODUCT BY FRACTIONAL DISTILLATION AT HIGH PRESSURE AND THEN AT LOW PRESSURE WHEREIN COMPRESSED AIR IS COOLED BY HEAT EXCHANGE WITH COLD STREAMS FROM THE PROCESS COMPRISING THE STEPS OF: COMPRESSING SAID AIR TO A HEAD PRESSURE OF 120-700 SPSIG, BEING HIGHER THAN SAID HIGH PRESSURE ISTILLATION, SELECTIVELY ADSORBING CARBON DIOXIDE AND ACETYLENE ATMOSPHERIC IMPURITIES FROM SAID COMPRESSED AIR PRIOR TO CRYOGENIC COOLING SO AS TO PROVIE PRECLEANED AIR, COOLING AND WORK EXPANDING A FIRST MINOR PORTION COMPRISING 1-20% OF SAID PRECLEANED AIR TO A PRESSURE BELOW SAID HIGH PRESSURE DISTILLATION AND RECOVERING REFRIGERATION FROM THE RESULTING COLD LOW PRESSURE GAS FOR THEPROCESS, COOLING AND LIQUEFYING A SECOND MINOR PORTION COMPRISING 1-20% OF SAID PRECLEANED AIR TO FORM LIQUID AIR AND DISTILLING SAID LIQUID AIR, WITH THE REFRIGERATION FOR SAID LIQUEFYING BEING PROVIDED BY SAID COOL STRAMS FROM THE PROCESS WHICH ARE THEREBY PARTIALLY WARMED, PARTIALLY COOLING THE MAJOR PORTION OF SAID PRECLEANED AIR BY THE COLD STREAMS WHICH HAVE BEEN PARTIALLY WARMED, TO TEMPERATURE ABOVE THE TEMPERATURE OF THE COOLED AND LIQUEFIED SECOND MINOR PORTION, AND WORK EXPANDING THE PARTIALLY COOLED MAJOR PORTION TO THE PRESSURE OF SAID HIGH PRESSURE DISTILLATION SUCH THAT THE EXPANDED COLD FLUID IS SUBSTANTIALLY ENTIRELY GAS, AND FLOWING AT LEAST PART OF THE RESULTING COLD GAS TO SAID HIGH PRESSURE DISTILLATION, AND WITHDRAWING AT LEAST ONE LIQUID PRODUCT AND AT LEAST ONE GAS PRODUCT FROM THE DISTILLATION AND DISCHARGING SUCH LIQUID AND GAS PRODUCTS FROM THE PROCESS.
 2. A process according to claim 1, wherein said first minor portion of said precleaned air to be work expanded is a diverted part of the work expanded major portion of said precleaned air which is reheated prior to said work expanding to pressure below said high pressure distillation.
 3. A process according to claim 1 wherein said first minor portion of said precleaned air to be work expanded is cooled to an intermediate temperature, said major portion of said precleaned air to be work expanded is cooled to a lower-than intermediate temperature, and said minor and major portions are thereafter work expanded in parallel flow relation.
 4. A process according to claim 1, wherein the work expanded first minor portion is thereafter heat exchanged with the precleaned air for cooling thereof as one of said cold streams.
 5. A process according to claim 1, wherein said first minor portion is work expanded to the pressure of the low pressure distillation and thereafter passed to said low pressure distillation.
 6. A process according to claim 1 wherein 2-15% of the precleaned air is cooled and liquefied as said second minor portion.
 7. A process for the cryogenic separation of air into liquid oxygen and nitrogen products and gaseous nitrogen product by fractional distillation at high pressure and then at low pressure wherein compressed air is cooled by heat exchange with cold streams from the process comprising the steps of: compressing said air to head pressure of 120-700 psig., being higher than said high pressure distillation; selectively adsorbing carbon dioxide and acetylene atmospheric impurities from said compressed air prior to cryogenic cooling so as to provide precleaned air; cooling and work expanding a first minor portion comprising 1-20% of said precleaned air to pressure below said high pressure distillation as cold low pressure air and heat exchanging same with said precleaned air as one of said cold streams, and releasing the so-warmed low pressure air from the process as excess air; cooling and liquefying a second minor portion comprising 1-20% of said precleaned air to form liquid air and distilling said liquid air, with the refrigeration for said cooling and liquefying being provided by said cold streams from the process which are thereby partially warmed; partially cooling the major portion of said precleaned air by the cold streams which have been partially warmed, to temperature above the temperature of the cooled and liquefied second minor portion, and work expanding the Partially cooled major portion to the pressure of said high pressure distillation such that the expanded cold air is substantially entirely cold gas; introducing a larger part of said cold gas to said high pressure distillation for partial separation threin; heat exchanging a smaller part of said cold gas with said second minor portion as one of said cold streams for said cooling and liquefying thereby reheating said smaller part, and employing the reheated smaller part as said first minor portion of precleaned air to be work expanded; withdrawing liquid oxygen and liquid nitrogen products from the distillation as separate streams and discharging such liquid products from the process; discharging gaseous nitrogen product from the distillation and heat exchanging same with said precleaned air as another of said cold streams for said cooling thereof, and recovering the so-warmed gaseous nitrogen product; and discharging gaseous nitrogen waste from the distillation and heat exchanging same with said precleaned air as still another of said cold streams for said cooling thereof, and releasing the so-warmed gaseous nitrogen waste from the process.
 8. A process according to claim 7 wherein said excess air is heated and flowed through adsorbent beds for desorption and removal of atmospheric impurities deposited therein during the air precleaning.
 9. A process for the cryogenic separation of air into liquid oxygen product and gaseous oxygen and nitrogen products by fractional distillation at high pressure and then at low pressure wherein compressed air is cooled by heat exchange with cold streams from the process, comprising the steps of: compressing said air to head pressure of 120-700 psig., being higher than said high pressure distillation; selectively adsorbing carbon dioxide and acetylene atmospheric impurities from said compressed air prior to cryogenic cooling so as to provide precleaned air; cooling and liquefying a first minor portion comprising 1-20% of said precleaned air at said head pressure by heat exchange with said cold streams to form liquir air, and distilling said liquid air for separation thereof; partially cooling a second minor portion comprising 1-20% of said precleaned air by heat exchange with said cold streams from the process, work expanding the partially cooled second minor portion to the pressure of the low pressure distillation and passing the work expanded air thereto for separation; cooling the major portion of said precleaned air by heat exchange with said cold streams from the process to lower temperature than said partially cooling, work expanding the cooled major portion to the pressure of the high pressure distillation such that the expanded cold fluid is substantially entirely gas, passing a major part of the work expanded major portion to the high pressure distillation for partial separation therein, heat exchanging a minor part of the work expanded major portion with cold gaseous oxygen product from the low pressure distillation thereby liquefying said minor part and partially warming said gaseous oxygen product, introducing the liquefied minor part to the distillation for separation therein and heat exchanging the partially warmed gaseous oxygen product with said precleaned air as one of said cold streams; withdrawing liquid oxygen from the distillation and discharging such liquid from the process as product; discharging gaseous nitrogen from the distillation, heat exchanging same with said first minor portion of precleaned air as said cold stream for liquefying thereof and heat exchanging the partially warmed gaseous nitrogen with said precleaned air as another of said cold streams, and recovering at least part of the so-warmed gaseous nitrogen as product.
 10. A process according to claim 1 wherein the major portion of said precleaned air is partially cooled to temperature above the atmospheric impurity dew point prior to said work expansion. 